Trimming of waveguide filters

ABSTRACT

A waveguide filter comprising a matching means for matching the filter. The waveguide filter comprises a housing comprising a cavity having a predetermined first volume. The matching means is in the form of a volume element having a predetermined second volume being matched to the first volume forming a predetermined volume to volume ratio. The matching means is fitted into the cavity in a fixed non-adjustable manner in relationship to the housing. The invention refers also to a method for manufacturing of such a waveguide filter.

TECHNICAL FIELD

The invention refers to a waveguide filter comprising a matching meansfor trimming the filter. The waveguide filter comprises a housingcomprising a cavity having a predetermined first volume. The matchingmeans is in the form of a volume element having a predetermined secondvolume being matched to the first volume forming a predetermined volumeto volume ratio. The invention also refers to a manufacturing process ofsaid waveguide filter.

BACKGROUND

In the field of waveguide filters it is known to use a housingcomprising a bottom portion, a top portion and side portions connectingthe bottom portion and the top portion. The housing comprises an inputport (also called an iris) for receiving an input signal and an outputport for transmitting an output signal. The filter receives the inputsignal and filters the input signal from certain frequencies andforwards the filtered signal as the output signal to a transmitting unitor transmits the output signal itself. The housing is designed dependenton the filter type, for example a band-pass filter or a low-pass filter.However, common for all filters are that the housing comprises one ormore cavities which together with corresponding trimming means isdesigned for filtering the input signal. The trimming means are in theform of one or a number of movable bodies corresponding to the number ofcavities. U.S. Pat. No. 5,349,316 teaches the use of movable bodies inthe form of trimming plungers or trimming screw.

U.S. Pat. No. 4,041,421 teaches the use of movable bodies in the form ofscrews being rotatably mounted in the top portion and each screwprotrude into one of said cavities. It is long known that the volume ofa trimming means body in the cavity is essential for the filterbehaviour. Therefore, when trimming the filter a predetermined inputsignal is fed to the filter whereafter the output signal is measuredwhile the screws are screwed into or out from the cavities until apredetermined and desired output signal pattern is achieved.

The need for the use of trimming means is to adjust the operatingfrequency of the filter to meet a desired frequency characteristic. Thetrimming means is either screwed into or out from the cavity in order tochange the capacitance of the waveguide filter. The trimming isnecessary because the position of the trimming means in the cavity andthe material of both the trimming means and the waveguide filter affectsthe capacitance. Furthermore, lack of dimension accuracy giving cavitiesnon-predictable sizes also gives a need for adjusting the trimming meansin order to compensate for the differences in order to get a number offilters to perform identically to another for the same filteringpurpose. Hence, every prior art waveguide filter have to be trimmedafter assembly which is a time consuming process step and therefore alsoa costly part of the manufacturing.

One problem with using screws is that the size of the housing decreaseswith the increase in frequency and for frequencies above 20 GHz thehousing is so small that the screws, the holes for the screws and thethreads for the screws become so small that the process of manufacturingthe same is difficult and costly. In addition to this problem, the smallsized screws are hard to handle because of their smallness and thetrimming of the filter becomes difficult and costly.

Thus, there exists a need for an improved waveguide filter being easierand cheaper to manufacture.

SUMMARY

The invention intends to remedy the above mentioned disadvantages andthe object is to produce a waveguide filter being easier and cheaper tomanufacture.

The object is met by the invention referring to a waveguide filtercomprising a matching means for trimming the filter. The waveguidecomprises a housing comprising a cavity having a predetermined firstvolume. The matching means is in the form of a volume element having apredetermined second volume being matched to the first volume forming apredetermined volume to volume ratio. The invention is characterised inthat the matching means is fitted into the cavity in a fixednon-adjustable manner in relationship to the housing.

Here, “fixed” refers to the matching means being permanently attached toa surface belonging to the housing or the wall. Furthermore,“non-adjustable” refers to the matching means having a predeterminedsecond volume that cannot be changed when the matching means has beenfixed in the cavity.

The invention refers also to a method for manufacturing the waveguidefilter and comprises the steps of:

-   -   manufacturing the housing separate from the matching means (2);    -   forming the housing to comprise a cavity having a predetermined        first volume;    -   forming the matching means in the form of a volume element        having a predetermined second volume being matched to the first        volume forming a predetermined volume to volume ratio;

The invention is characterised in that the method comprises the step offitting the matching means into the cavity in a fixed non-adjustablemanner in relationship to the housing.

One benefit of the invention is that the predetermined housing andpredetermined matching means gives the possibility to manufacture allparts separately with a high degree of repetition accuracy whereafterthe waveguide filter is assembled by attaching the matching means in thecavity in a predetermined position relative the housing. Hence, a numberof waveguide filters with predetermined performance may be manufacturedwithout having to trim every waveguide filter after assembly. Themanufacture therefore becomes simple and cheap and allows for rapidchanges in the production line with regard to different demands on thefilter. For example if the production line produces waveguide filterswith a predetermined bandwidth and a demand for a waveguide filter withdifferent bandwidth characteristics occurs, this demand may be met baysimply changing the matching means used to a pre-fabricated matchingmeans giving a different volume to volume ratio and thereby changing thebandwidth characteristics accordingly.

Further advantages of the invention will be explained below in thedetailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The below described embodiments are all different options for theinvention and may be combined within the scope of the appendedindependent claims.

In one embodiment of the invention, the housing only partly encompassesthe cavity. The housing is mounted onto a conductive surface of acarrier forming a wall closing the cavity. The enclosed cavity comprisesthe predetermined first volume, and the second volume is matched to thefirst volume forming a predetermined volume to volume ratio. Thematching means is fitted into the cavity in a fixed non-adjustablemanner in relationship to the housing and the wall.

Here “partly encompassing a cavity” refers to a three dimensionalstructure having one open side. The housing is formed such that the openside comprises a peripheral edge peripherally surrounding the cavity andintended to face the carrier surface. The peripheral edge has anextension in a plane and is intended to be mounted onto the conductivesurface having an extension in the same plane. The carrier surface thusforms the above mentioned wall against the peripheral edge and thuscloses the cavity upon application of the housing onto the carrier.

One benefit of this embodiment is that the carrier surface easily can bemanufactured in a predetermined and precise manner by known methods andthat the housing may be positioned and attached to the carrier surfacein a precise manner. Further advantages of this embodiment will becomeapparent below.

The carrier may be made from a number of materials, but a cost efficientchoice is the use of a plastic material which is known from circuitboards. The carrier surface advantageously comprises a metal layercoated onto the carrier. The metal layer may be formed by, for example,etching into a shape corresponding to the peripheral edge of thehousing. The housing is advantageously attached directly onto the metallayer by a suitable attachment method. One especially advantageousattachment method is soldering because the shape of the metal layercorresponds to the shape of the peripheral edge of the housing such thatthe liquid soldering material automatically aids in the positioning ofthe housing due to surface tension phenomena.

The matching means is designed dependent on a number of parameters, forexample choice of material, position in the cavity and shape, and otherparameters having an impact on the signal to be processed. The secondvolume therefore differs dependent on all the parameters and has to bechosen accordingly. However, the benefit of the invention lies in thepossibility to use the parameters and the first volume in order topredict the second volume and that the process of assembling thewaveguide filter therefore becomes simple and flexible.

The housing may have any suitable form for wave-guiding and may comprisecomplex features and details that affect waves travelling within thehousing. The housing may have a pyramidal form, i.e. a squared base withdecreasing square sections on top of each other starting from the baseand a top wall being planar with the peripheral side edge of the squarebase. The cavity thus has a corresponding shape with stepwise decreasinginner walls from bottom to top. Other shapes are also possible, but thelowest common denominator is that the housing has precise dimensioncharacteristics suitable for generating a standing wave. The precisedimension characteristics may be achieved by moulding the housing in asuitable material, for example plastics or zinc or the like and thenplating the material with an electrically conduction material such asgold, silver, cupper or the like.

One advantage of the invention lies in that the predetermined first andsecond volume gives a simple and flexible manufacturing process where aseparate matching means is chosen for a certain housing and carrier whenit is time for assembly. The great benefit of the invention is thatdifferent batches may be manufactured with different dimensions of thehousing, the carrier and the matching means. Each batch comprises anumber of essentially identical housings, a number of essentiallyidentical walls (i.e. carrier surfaces) and a number of essentiallyidentical matching means all which may be manufactured separately givinga low cost because the same casting mould or the same tool may be usedfor manufacturing a great number of the essentially identical devices ineach batch. Another cost benefit is due to the fact that the waveguidedoes not have to be trimmed as the previously known waveguide filters.The previously known trimming means is in the form of trimming screws,or the like, that has to be adjusted after assembly of the waveguide.The adjustment is time consuming and therefore costly. The invention, onthe contrary, does not have to be trimmed but everything ispredetermined on assembly. The flexibility of the invention lies inthat, for example, an operator may build different waveguide filters bychoosing the appropriate items from the different batches and thenassembly them in an easy and cost-efficient way without the need fortrimming.

In one preferred embodiment of the invention the matching means isdirectly attached to the carrier surface. One advantage of thisembodiment is that the housing and the carrier may be manufacturedindependently of the matching means as long as the housing and theconductive carrier surface are manufactured with a high degree ofrepeated dimensional accuracy, i.e. that a series of housings and aseries of carrier surfaces respectively are essentially identical for acertain predetermined frequency. The independent manufacturing of thehousing and the carrier surface gives two key components that may beused together with different matching means. Preferably, the solderingmethod described above is used where the matching means is positionedonto the carrier surface in a predetermined position and where thesoldering material automatically centres the matching means in itspredetermined position.

The matching means are also manufactured independently with differentvolume dimensions and with a high degree of repeated dimensionalaccuracy. This gives the advantage that the same housing and carrier maybe used for different filters by simply attaching different matchingmeans to the carrier surface depending on the desired volume to volumeratio. As stated above the volume to volume ratio gives the operatingparameters for the filter.

When manufacturing the housing, the batches of housings may becontrolled by a random pick of housings from the batch whereafter thehousings are tested without the matching means. The testing is performedby transmitting a known signal into the housing and measuring the signalon exit from the housing. The testing reveals information on thehousings ability to generate standing waves and in what frequencies. Thebatch may then be labelled accordingly and may be matched with asuitable matching means with a predetermined second volume forproduction of a waveguide filter with predetermined features. Thisdiffers from prior art since the present invention allows measurementson only a few housing in order to get a large number of waveguidefilters, but in prior art every waveguide filter has to be trimmed sincethe trimming means in prior art do not have a predetermined secondvolume, but the second volume is experimentally matched to the firstvolume by hands on trimming after assembly.

According to one embodiment of the invention, the matching means is inthe form of a separate solid object being attached to the carriersurface via attachment means. The attachment means may be in the form ofglue or a solder material such as, for example, led or tin. Theattachment means is advantageously in a fluid state and positioned onthe carrier surface. The matching means is thereafter positioned on thefluid attachment means. The fluid state of the attachment means has theadvantage that surface tension phenomena affects the matching means insuch a way that the matching means is centred with regard to the lateralextent of the attachment means. When positioning the attachment means, apositioning means may be used for delimiting the fluid attachment meansto a certain area. The positioning means thus hinder the liquid fromexiting a predetermined area.

The positioning means is preferably in the form of a circular ring thathinders the fluid attachment means from exiting the area and thecircular form also helps to distribute the attachment means evenlywithin the area. The positioning means have another shape than circular,for example square, rectangular, oval or the like, as long as it ispossible to predict the position of the matching means with regard tothe shape of the positioning means.

The positioning means may be in the form of a solder stop when using asoldering method, and may be in the form of a tape strip or anythingsimilar that may hinder the liquid from exiting the area. If thepositioning means builds substantial volume, the positioning means alsohas to be taken into consideration when calculating the second volume.

When the matching means is positioned and centred, the attachment meansis solidified. The solidification may be achieved by cooling down a hotattachment means, for example, when soldering or a when using a hotadhesive. The solidification may also be achieved by use of a non-heatedadhesive that solidifies after a certain time period due to itscomposition, or that may be solidified by use of an external means, forexample, an ultraviolet lamp etc. The attachment means has to be takeninto consideration when calculating the second volume since theattachment means builds volume unless the attachment means has athickness greater than zero.

The advantage of this embodiment lies in that the matching means can bemanufactured with different second volumes as long as the surface of thematching means intended to be attached to the carrier surface has thesame size for all volumes. The benefit of being able to switch betweenmatching means having different second volumes has been explained aboveand will be further explained below.

According to another embodiment, the matching means is in the form of aseparate solidified object being attached to the carrier by thesolidified object being formed onto the carrier in liquid form and thenbeing solidified. A positioning means may be used for this embodimenttoo for the reasons stated above. The advantage of this embodiment isthat the dimension, i.e. volume, of the matching means may be decided insitu when the matching means is to be attached to the carrier by addingdifferent amount of liquid material to the target area. This embodimentthus has the same advantages as the above mentioned embodiment with asolid object attached by attachment means and solves the stated problemof a more efficient manufacturing of a waveguide filter.

The liquid material may be a solidifiable adhesive such as atwo-component glue or plastic that solidifies after a predetermined timeperiod due to composition itself. The liquid material may also be amaterial that solidifies after being treated by an external source, forexample ultra violet light or laser or microwave or any other suitableexternal source. Examples of such materials are a number of differentplastics, rubber, ceramics and the like.

The non-conductive materials may be made, at least partly, conductive byaddition of a suitable electrically conductive material. The reason forusing an electrically conducting material is that it minimises thelosses in the waveguide filter. The non-conductive materials havedifferent dielectric properties which affect the wave to be processedand therefore affect the second volume such that the greater thedielectric constant the lesser the second volume.

The liquid material may also be in the form of a metal that has beenliquefied by, for example, heating. The liquid metal solidifies when ithas been cooled down to a temperature specific for the metal inquestion. The metal may be in the form of, for example, tin, silver,gold, led, zinc, cupper, etc. and suitable combinations of the metals,both as alloys and in the form of plating. One example of a suitablelayer structure in a matching means is the use of plastic forming themain part of the volume and a metal plating on the solid plasticsurface. The metal may be any suitable metal, for example cupper, silveror gold. A combination of zinc and cupper is also a feasible solutiongiving the stated advantages. Furthermore

The solidified material may have a dome shape or any other shape thatcomes natural in a solidification process in a system affected bygravity. Please note that gravity is not a prerequisite for the process,but is only the most normal environment in a manufacturing plant. Hence,the process may be done in a non-gravity environment with a differentshape of the solidified matching means as a consequence.

The matching means may also be built from a number of solidified layers.when one layer has been solidified the next layer is applied onto thesolidified layer and so on until the matching means has been shape. Oneadvantage of this embodiment is that the shape of the matching means maybe modelled into a desired shape by altering the height and width ofeach layer.

According to one embodiment of the invention the matching means is fixedin the cavity by being directly attached onto an inner surface of thehousing. Any of the above described methods may be used and the abovestated advantages of the invention using predetermined volume to volumeratio are achieved. However, the process of attaching the matching meansin the housing is somewhat more complicated than the attachment onto thesurface of the carrier, why the latter alternative is a betteralternative with regard to the manufacturing process.

The inventive waveguide filter may advantageously be used in a front endbranching unit for microwaves and as a filter in a chain of signalprocessing and a number of other previously known fields.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will below be explained in view of a number of drawingswhere:

FIG. 1 schematically shows a top view of a waveguide filter according tothe invention.

FIG. 2 schematically shows a cross-sectional side view along line A-A inFIG. 1 of a waveguide filter according to a first embodiment of theinvention, and where;

FIG. 3 schematically shows a cross-sectional side view of along line A-Ain FIG. 1 of a waveguide filter according to a second embodiment of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 schematically shows a top view of a waveguide filter 1 accordingto the invention. The waveguide 1 according to FIG. 1 comprises threematching means 2 and a housing 3 partly encompassing three cavities 4.The housing 3 is mounted onto a conductive surface 5 of a carrier 6forming a wall 7 closing the cavity 4. The matching means 2 is in theform of a volume element. Each enclosed cavity 4 has a predeterminedfirst volume and the corresponding matching means 2 has a predeterminedsecond volume being matched to the first volume forming a predeterminedvolume to volume ratio. Each matching means 2 is fitted into thecorresponding cavity 4 in a fixed non-adjustable manner in relationshipto the housing 3 and the wall 7. The invention is of course not limitedto the use of three cavities, but may comprise any number of cavitiesranging from 1 and up.

FIG. 1 shows that the carrier 6 surface 5 is partly coated with apositioning means 8. The positioning means 8 is in the form of a ringand the matching means 2 is positioned in the centre of the ring.

FIG. 1 shows that the housing 3 comprises an input port 9 and an outputport 10 for receiving and transmitting a microwave signal. The housing 3in FIG. 1 also comprises four side walls 11, a top wall 12, and twopartition walls 13. The partition walls 13 are positioned in the housingsuch that they delimit a space in the housing and thereby forming thethree cavities 4. Each partition wall 13 comprises an intermediate port14 for guiding the signal in a direction from the input port 9 to theoutput port 10 via the matching means 2.

The volume to volume ratio is chosen dependent on the use of thewaveguide filter, i.e. if it is to be used as a band-pass filter,low-pass or high-pass filter. Furthermore, the second volume and thusthe volume to volume ratio depends on a number of parameters, namely:the material in the matching means 2; the position of the matching means2; the shape of the matching means 2; the dielectric constant of thematching means 2; and the position and size of the input port 9, outputport 10 and the intermediate ports 14.

FIG. 1 shows that the housing 3 comprises a cavity 4 with a square basewith a width W0 a length L. FIG. 1 also shows that the in-port 9 has awidth W1 and that the out-port 10 has a width W2. Furthermore, thematching means 2 have been depicted with different heights ht in thedifferent cavities. This is only to show that the volume to volume ratiomay easily varied according to the invention by simply using differentmatching means 2 with different heights.

FIG. 2 schematically shows a cross-sectional side view along line A-A inFIG. 1 of a waveguide filter according to a first embodiment of theinvention. FIG. 2 shows that the housing 3 and each cavity 4 have arectangular cross-section where the cavity has a height h. The inventionis not limited to cavities 4 being square or rectangular, but othershapes are possible for generating a standing wave. FIG. 2 shows thatthe matching means 2 are in the form of cylindrical elements having aradius Rt and being attached to the surface 5 of the carrier 6. Thematching means may of course have other forms, for example cone shaped,pyramidal, spherical, ellipsoidal, cylindrical, or any other shapesuitable for the stated purpose of matching in a waveguide filter Thematching means 2 are, in FIG. 2, in the form of solid objects beingattached directly onto the surface 5 by a suitable method, for examplethe use of an adhesive or by soldering.

FIG. 2 shows that the positioning means 8 has a height and thus a volumethat has to be taken into consideration when calculating the volume tovolume ratio between the first volume and the second volume. It ispossible to consider the volume of the positioning means 8 being part ofeither the first volume or being a part of the second volume. However,when deciding the volume of the positioning the same parameters have tobe taken into consideration as when deciding the second volume, i.e. theposition, the material and the form.

Below is an example of a waveguide band-pass filter for a 23 GHz signaland a corresponding band-pass width. In order to simplify thedescription of the example, the example is described with reference toFIGS. 1-2. The example shall not be seen as limiting for the invention,but the invention may be varied within the scope of the claims.

h=4 mm; ht=0-5 mm; L=6-9 mm; W0=2-4 mm; W1=W2=10.7 mm; rt=1-3 mm

For these parameters, the waveguide filter 2 performs as a band-passfilter with a predetermined band-pass width dependent on ht, L and W0.

FIG. 3 schematically shows a cross-sectional side view of along line A-Ain FIG. 1 of a waveguide filter according to a second embodiment of theinvention. FIG. 3 is identical to FIG. 2 apart from the matching meansthat have different form than in FIG. 1. The matching means in FIG. 3 ismanufactured by use of a solidifiable liquid which has been positionedonto the carrier 5 surface 6 in its liquid form. The liquid has thensolidified into a solid state. In FIG. 3 the matching means 2 are domeshaped due to the fact that the surface tension of a liquid togetherwith the gravity automatically gives such a form. The matching means mayof course have other forms when using a different manufacturing process,for example cone shaped, pyramidal, spherical, ellipsoidal, cylindrical,or any other shape suitable for the stated purpose of matching in awaveguide filter. In FIG. 3 the matching means 2 are shown withdifferent sizes for the same reason as in FIG. 2.

The invention is not limited to the embodiments in FIGS. 1-2, but may bevaried within the scope of the claims. For example, the matching meansmay be attached to the inside surface of the housing and the positioningmeans may then be applied onto the inside surface accordingly.

1. A waveguide filter comprising: a housing having a cavity with apredetermined first volume; a matching means in the form of a volumeelement having a predetermined second volume being matched to the firstvolume forming a predetermined volume to volume ratio, wherein thematching means is fitted into the cavity in a fixed non-adjustablemanner in relationship to the housing, and the matching means is in theform of a solidified object being attached to the housing by thesolidified object being formed into the housing surface in liquid formand then being solidified.
 2. The waveguide filter according to claim 1,wherein the housing partly encompasses the cavity, the housing beingmounted onto a conductive surface of a carrier forming a wall closingthe cavity.
 3. The waveguide filter according to claim 2, wherein thematching means is directly attached to the carrier surface.
 4. Thewaveguide filter according to claim 2, wherein the matching means beingattached to the carrier surface via attachment means being part of thesecond volume of the matching means.
 5. The waveguide filter accordingto claim 1, wherein the matching means being attached to the housing viaattachment means being part of the second volume of the matching means.6. The waveguide filter according to claim 2, wherein the carriersurface is partly coated with a positioning means for hindering theliquid from exiting a predetermined area.
 7. A method for manufacturinga waveguide filter having a matching means for matching the filter, themethod comprises the steps of: manufacturing a housing separate from thematching means; forming the housing to have a cavity having apredetermined first volume; forming a matching means in the form of avolume element having a predetermined second volume being matched to thefirst volume forming a predetermined volume to volume ratio, wherein thematching means is formed as a solidified object attached to the housingby being formed onto the housing surface in liquid form and then beingsolidified; and fitting the matching means into the cavity in a fixed,non-adjustable manner in relationship to the housing.
 8. The method formanufacturing a waveguide filter according to claim 7, wherein thehousing partly encompasses the cavity and is mounted onto a conductivesurface of a carrier forming a wall closing the cavity.
 9. The methodfor manufacturing a waveguide filter according to claim 8, wherein thematching means is directly attached to the carrier surface.
 10. Themethod for manufacturing a waveguide filter according to claim 8,comprising the further steps of: attaching the matching means to thecarrier surface via an attachment means being part of the second volumeof the matching means.
 11. The method for manufacturing a waveguidefilter according to claim 7, wherein the matching means attached to thehousing via an attachment means being part of the second volume of thematching means.
 12. The method for manufacturing a waveguide filteraccording to claim 8, wherein the carrier surface is partly coated witha positioning means for hindering the liquid from exiting apredetermined area.